Compact neutron source



Sept. 17, 1963 A. J. GALE ETAL COMPACT NEUTRON SOURCE Filed Feb. 10, 1958 l y y A M United States Patent 3,104,322 COMPACT NEUTRON SOURQE Alfred J. Gale, Lexington, and Richard 3. Connor, Medway, Mass, assignors to High Voltage Engineering Corporation, Burlington, Masa, a corporation of Massachusetts Filed Feb. 10, 1958, Ser. No. 714,303 4 Claims. (Cl. 250-34.5)

This invention relates to neutron sources, and in particular to a compact neutron source adapted for use in regions having limited volume and to which only a limited amount of power can be delivered, such as boreholes. Broadly stated, the invention comprises a neutron source wherein ions such as protons, deuterons or tnitons are produced by a field emission ion source and accelerated toward a suitable target for the production of neutrons. The ion source of the invention comprises a member, such as a wire, having a sharp point and being composed of any metal which absorbs isotopes of hydrogen, such as titanium, palladium or nickel. A heater coil may be provided to heat the wire, but this may not be necessary.

The invention may best be understood from the following detailed description thereof having reference to the accompanying drawing in which:

FIG. 1 is a View in longitudinal central section, partly broken away, of a neutron source constructed in accordance with the invention; and

FIG. 2 is a detail showing a modified construction of the target of the apparatus shown in FIG. 1.

Referring to the drawing and first to FIG. 1 thereof, the device therein shown may be constructed along the general lines disclosed in a co-pending application, Serial No. 515,435, filed June 14, 1955, now US. Patent No. 2,907,884, issued Oct. 6, 1959, and assigned to the assignee of the present invention. The entire neutron source constructed in accordance with our invention may be enclosed within a housing 1 of generally cylindrical configuration, and in the device shown in FIG. 1 the housing 1 is particularly adapted to be lowered down a borehole. The housing 1 is of conductive material and is at ground potential. As is described more fully in the aforementioned co-peuding application, the entire power for operating the neutron source is derived through a single electric motor (not shown) which drives the upper pulley -2 of an electrostatic beltatype generator 3. The operation of an electrostatic belt-type generator is Well known, and it is sufficient herein to state that negative charge is deposited on an endless insulating belt 4 by a corona discharge from the charge-spray points 5, and after the belt 4 has carried this negative charge to within a hollow electrode 6, the negative charge is removed from the belt 4 by the charge-collector points 7. The belt 4 is supported upon the upper pulley 2 and a lower pulley 8, which is located within the hollow electrode 6.

The interior of the housing 1 is divided into two main compartments. The neutron source proper is housed in the lower compartment 9. The upper compartment houses the voltage generator '3 which is necessary to operate the neutron source. In operation the uppercompartment 10 is filled with a gas under pressure in order to insulate the various high-voltage parts of the apparatus from the grounded housing 1. The lower compartment 9 is highly evacuated during assembly of the device, and in operation the lower compartment 9 will be filled with the gaseous isotope :of hydrogen, which is emitted from the ion source to he described hereinafter, at very low pressure. These two compartments are therefore hermetica'lly closed off from the surrounding atmosphere and from each other.

As noted, the upper compartment 10 is pressurized "ice with an insulating gas in order that high voltage may be maintained within the small space. In accordance with the invention, any voltage source may be used to raise the potential of the electrode 6 to a high negative potential of, for example, 250 kilovolts. However, by way of example, an electrostatic belt-type generator 3 is shown in FIG. 1. A target 11 adapted to produce neutrons upon bombardment with positive ions is afiixed to an extension 12 'of the hollow electrode 6 so as to be maintained at the high negative potential of the hollow electrode 6.

In the lower compartment 9 ions are produced by an ion source 13 and accelerated onto the target 11. As stated hereinbefore the ion source 13 is of the field-emission type and comprises a suitable member, such as a wire 14, having a sharp point 15 and being composed of titanium, palladium, nickel or other suitable metal which absorbs isotopes of hydrogen. Prior to assembly, the wire 14 is impregnated with an isotope of hydrogen in gaseous form by placing the wire in a chamber containing an isotope of hydrogen under pressure. Provided that the wire is kept at a sutficiently low temperature after removal from the high pressure chamber, the gas will be stored therein and will not escape. The wire then contains a sufficient reservoir of gas to enable it to operate as an ion source for an extended period of tim Referring again to the neutron source shown in FIG. 1, a grid-like member which may comprise a metal disk 16 having an aperture 17 is placed near the point 15 of the wire 14. The grid 16 is connected to a pulsed voltage supply 18 which periodically raises the potential of the grid 16 with respect to the wire 14, which is grounded, to a potential of several kilovolts negative. The resultant strong electric field in the vicinity of the point 15 is at least of the order of 10 volts per centimeter and pulls positive ions away from the tip 15. The high negative potential on the target 11 then accelerates these positive ions onto the target 11, where neutrons are produced. The electric field at the tip 15 of the wire 14 may itself be sufiicient to cause ion extraction. If not, the release of ions may be augmented by heating the wire 14 by means of a heating coil 19 Whose temperature is raised by passing a current therethrough from a power supply 20. The power supply 20 and the pulsed voltage supply 18 may be energized in any suitable manner, as by means of leads extending to the surface of the borehole; in FIG. 1, for the sake of simplicity, the power supply 20 and the pulsed voltage supply 18 are shown merely diagrammatically.

The present invention provides several advantages over compact neutron sources heretofore in use. First, it provides increased simplicity over neutron sources utilizing more conventional types of ion source.

Second, no neutral gas flow is required. As hereinbefore noted, the wire contains a relatively large reservoir of gas and no external gas source is required. Other types of ion sources in addition to producing the necessary ions also produce a flow of neutral-s gas into the ion-acceleration region, which gas fiow must be taken care of either by pumping or else by providing means to enable the ion source to operate at very low gas pressure.

Third, the device constructed in accordance with the invention eliminates the necessity for balancing currents very carefully. Such current balancing is required in more conventional designs where the voltage generator for the ion source may short-circuit the high-impedance column of the ion accelerating voltage generator. That is to say, where the ion source includes a gas discharge actuated by a voltage source, there is a high current in the ion source; and when the magnitude of this ion-source current becomes much greater than the ion-beam current, the ion-accelerating voltage drops to zero. As a result it is necessary to provide such conventional ion sources with means for limiting the current in the ion-source discharge or for compensating therefor. The field-emission source of the invention, on the other hand, provides an inherently constant voltage apparatus by Virtue of the fact that it acts as a corona stabilizer. That is to say, it takes a very great change in ion current to produce even a small change in voltage between the point 15 and the target 11.

Fourth, no power is required in the ion source of the invention except for a small resistive loss in the heater coil 19.

Pi "th, the invention also simplifies focusing problems because it provides a point source, unlike conventional gasdischarge ion sources.

Sixth, the ions emitted by the ion source of the invention are mostly monatornic; this tends to cause the emitted neutrons to be monoenergetic Without the need for beam-analyzing devices. The ions are mostly monatomic because the gas must diffuse through the material of the wire 14 in the form of protons, deuterons or tritons, for the hydrogen molecule is much too big to diffuse through the wire 14.

Referring now to FIG. 2, therein is shown means for reducing the load on the voltage generator 3 caused by the secondary emission of electrons. Such secondary electrons are produced from the target 11 b the bombarding thereof by the positive ions. A metal tube -21 is provided at the target 11 and has a depth which should be about three times its internal diameter. This metal tube 21 acts as a Faraday cage and collects most of the secondary electrons which are emitted by the target 11. In this way not only is the load on the generator 3 reduced, but the stray radiation produced by the bombardment of various parts of the apparatus by the secondary electrons is also minimized.

Having thus described the principles of the invention toethcr with an illustrative embodiment thereof, it is to be understood that although specific terms are employed they are used in a generic and descriptive sense and not 4 for purposes of limitation, the scope of the invention being set forth in the following claims.

We claim:

1. A compact neutron source adapted for use in regions having limited volume and to which only a limited amount of power can be delivered, such as boreholes, comprising in combination: an ion source, a target separated therefrom in an ion-acceleration region adapted to produce neutrons upon being bombarded by positive ions from said ion source, and an electrostatic belt-type generator adapted to maintain a negative voltage at said target with respect to said ion source, said ion source comprising: a solid metallic member Wholly within the ion-acceleration region and having a sharp point and impregnated with at least one isotope of hydrogen whereby a reservoir of said isotope is formed within said metallic member b virtue of such impregnation, and means for producing a strong electric field at said point and so oriented as to extract positive ions therefrom out of said reservoir, said point electrically facing said target so as to provide corona stabilizing action.

2. A compact neutron source according to claim 1, wherein said metallic member comprises titanium.

3. A compact neutron source according to claim 1, wherein said metallic member comprises palladium.

4. A compact neutron source according to claim 1, wherein said metallic member comprises nickel.

References Cited in the file of this patent UNITED STATES PATENTS 2,211,668 Penning Aug. 13, 1940 2,251,190 Kallmann et al July 29, 1941 2,417,797 Hipple Mar. 18, 1947 2,524,031 Arps Oct. 3, 1950 2,712,081 Fearon et al June 28, 1955 2,809,314 ierb Oct. 8, 1957 2,842,695 Goodman July 8, 1958 2,852,696 Johnson Sept. 16, 1958 2,905,826 Bonner et al Sept. 22, 1959 

1. A COMPACT NEUTRON SOURCE ADAPTED FOR USE IN REGIONS HAVING LIMITED VOLUME AND TO WHICH ONLY A LIMITED AMOUNT OF POWER CAN BE DELIVERED, SUCH AS BOREHOLES, COMPRISING IN COMBINATION: AN ION SOURCE, A TARGET SEPARATED THEREFROM IN AN ION-ACCELERATION REGION ADAPTED TO PRODUCE NEUTRONS UPON BEING BOMBARDED BY POSITIVE IONS FROM SAID ION SOURCE, AND AN ELECTROSTATIC BELT-TYPE GENERATOR ADAPTED TO MAINTAIN A NEGATIVE VOLTAGE AT SAID TARGET WITH RESPECT TO SAID ION SOURCE, SAID ION SOURCE COMPRISING: A SOLID METALLIC MEMBER WHOLLY WITHIN THE ION-ACCELERATION REGION AND HAVING A SHARP POINT AND IMPREGNATED WITH AT LEAST ONE ISOTOPE OF HYDROGEN WHEREBY A RESERVOIR OF SAID ISOTOPE IS FORMED WITHIN SAID METALLIC MEMBER BY VIRTUE OF SUCH IMPREGNATION, AND MEANS FOR PRODUCING A STRONG ELECTRIC FIELD AT SAID POINT AND SO ORIENTED AS TO EXTRACT POSITIVE IONS THEREFROM OUT OF SAID RESRVOIR, SAID POINT ELECTRICALLY 